In marine based seismic acquisition systems, one or more vessels tow seismic sources and seismic receivers. A conventional marine based seismic acquisition system 10 is illustrated in FIG. 1. As illustrated, a single survey vessel 21 tows one or more streamers 22 that may extend over kilometers behind the vessel. Each streamer includes seismic receivers disposed along its length. One or more source arrays 23 may also be towed by the survey vessel 21 for generating seismic wave. Conventionally, the source arrays 23 are towed in front of the streamers 22 when viewed in accordance with a direction of travel of the survey vessel 21 as indicated by arrow A. The seismic waves generated by the source arrays 23 propagate downwards, penetrate the seafloor and are reflected by a reflecting structure at an interface between different layers of the subsurface. The reflected seismic waves propagate upwards and are detected by the seismic receivers or sensors mounted on the streamers 22 towed by the survey vessel 21.
When multiple streamers are used for seismic data acquisition, deflector devices 25, e.g., are used to maintain streamer separation. Lead-in devices 24 or lead-in cables are used to connect the vessel to the deflectors and each one of the streamers. In order to maintain (control) the streamer depth at the front of the array, floats or buoys 26 are normally attached in between or close to the connection point between the lead in cables and the streamer. As illustrated in FIG. 2, each buoy is located on the surface of the water 30 and is attached to the streamer or lead-in through a separate cable, chain or rope 28. The buoy provides sufficient buoyancy of the front of the streamer array and prevents the front of the streamer array from diving further below the surface of the water. While mechanisms are known to adjust the depth of front of the streamer array through adjustments in the length of the cable 28, the buoy remains on the surface of the water. In this conventional arrangement, the buoy and lead-in are susceptible to interference with the towed sources, and the buoy can also contact, for example, ice flows or other objects on the surface of the water.
Therefore, a need exists for a towed streamer and buoy arrangements that can provide the desired depth for the front of the streamer array while eliminating surface buoys. Such as system would also facilitate selective adjustment of this depth and retrieval of the streamer array.